You notice this first as Shields rehearses her new NBC sitcom "Suddenly Susan," blocking scene after scene with her fellow actors. Then you notice something else.

She's funny.

Very funny.

Who knew?

The country (well, 66.3 million people, to be exact) glimpsed her comedic talents when she popped up on the much-hyped post-Super Bowl hourlong "Friends" episode in January that also featured Julia Roberts, Chris Isaak and Jean-Claude Van Damme.

This wasn't the Brooke Shields that America was used to--but a delusional fan so obsessed with soap star Joey (Matt LeBlanc) that she resorts to licking his hands to show her devotion.

But it started the wheels spinning in the heads of NBC execs.

In a series of events straight out of a Jackie Collins novel, the 31-year-old Shields soon had her own sitcom. She plays an under-confident, early-30-ish copy editor at a trendy San Francisco magazine who ditches her rich, handsome fiance at the altar, then begs her quirky boss (Judd Nelson) for her old job back. Impressed at her chutzpah (even though her ex is his brother), he makes her a columnist. The role gives her plenty of opportunity for physical humor, i.e. sliding off bar stools and singing karaoke while completely blotto.

NBC's belief in the show is evident in its platinum time slot: Thursday nights at 9:30, after "Seinfeld," starting this week.

No wonder Shields is bouncing around the set like a puppy dog just rescued from the pound. During a hectic day of blocking scenes for the second episode, she sprints over to an empty chair to talk before the assistant director crooks his finger at her to come back. She's dressed in a schlumpy sweatshirt, jeans and tennis shoes. A flash of light bounces off her left hand--it's a hefty rock, courtesy of her fiance, tennis star Andre Agassi.

"It was such an isolated moment," she recalls of the "Friends" episode. "To me, it was simply a guest spot, and I felt honored to be a part of it. My first question when I heard that [the part] was offered was 'Am I playing myself?' Because if the answer was yes, I would have had to turn it down. Why perpetuate anything when it was the antithesis of what I wanted to delve into and work on as an actress? But it was the first glimmer that I had that I would enjoy an environment like this."

Though film was always her medium of choice, Shields was becoming aware of television's renaissance. The concept of "Suddenly Susan," and the character, appealed to her.

"I felt like she had an opportunity to have a voice for women my age," she explains. "[Until you reach 30] you're dealing more with how to survive than who you really are, and I think this period for me and a lot of my friends has been one of self-discovery. . . . There's a scene with her grandmother where she says, 'You've got to find out what makes Susan Susan.' . . . I've been asking myself these questions for about three years, and I think that's what I identified with. And whatever was humanly pathetic about her spoke to me."

Suffice to say that pre-"Friends," Shields' reputation as a comedic actress was pretty much nonexistent. Though her dramatic roles have been varied, till now she has never shown the world that she could pull off a successful pratfall.

"I think I've always been to a certain extent the self-deprecating one," she says, tugging back a hank of hair. "I was the one willing to be silly or make the joke, ever since I was in school. . . . but it's something that I've never professionally explored, and I've never had the opportunity or encouragement. . . . Humor was always an outlet for me--it's the easiest defense and the easiest diffusion, whether from fear or just breaking the ice. And [doing this show] is cathartic, to be able to do it every day and watch people laugh."

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This is the latest chapter in a career remarkable for longevity if not for consistency. Shields has been in front of a camera since she was in diapers, starting with modeling (her early-teen "Nothing comes between me and my Calvins" campaign will be forever etched in our collective commercial subconscious) and segueing into films that ranged from impressive ("Pretty Baby") to silly ("Blue Lagoon") to not-even-a-rental ("Brenda Starr"). She received good notices for her stint as tough chick Rizzo in "Grease" on Broadway two years ago and as a stalking victim in the 1993 CBS docudrama "I Can Make You Love Me: The Stalking of Laura Black."

Even while doing hard time in academia (Princeton), she's never been too far out of the gossip loop.